Activated platelets and platelet releasates enhance the rate of FXI activation by α-thrombin. In all panels, initial rates of FXI activation were quantified at 37°C in reactions containing 30nM FXI, 5 to 20nM α-thrombin, and activated platelets or platelet releasate. (A) Dose response for platelet releasates from donor A in supporting FXI activation by 5nM (▴), 10nM (■), or 20nM (●) α-thrombin. Controls included boiled releasate (⊙) incubated with FXI and 20nM α-thrombin; and releasate preincubated with 250 μg/mL EcPPXc (○) or predigested with 70 μg/mL rPPX1 (ø) and then allowed to react with FXI and 20nM α-thrombin. (B) Activated platelets and releasates enhance FXI activation by thrombin. Initial rates of FXI activation by 20nM α-thrombin were quantified in the presence of 10-fold diluted platelet releasate (solid bars) or the same dilution of activated platelets plus releasate (open bars) from donor B, with or without preincubation with 250 μg/mL EcPPXc. Controls include FXI incubated with activated platelets (or releasate) from donor B but without α-thrombin, and FXI incubated with α-thrombin but without platelets or releasate. Data are mean ± SE (n = 3–4) from 2 separate donors. Platelets from donor A were activated at a concentration of 1.6 × 10⁷/μL, whereas platelets from donor B were activated at a concentration of 5.3 × 10⁶/μL.